The surface irregularities of a workpiece or part may be divided into three categories, namely: formness, waviness, and roughness. Formness is the general shape or outline of the workpiece, waviness is a measure of gradual variations in a workpiece surface such as undulation, camber and the like, while roughness is a measure of very small surface irregularities, typically less than 0.030 inch. As parts tolerances have become tighter over the years, the importance of surface roughness to the functionality of a physical surface has been well recognized. It is known, for example, that roughness affects such surface characteristics as coefficient of friction, nature and degree of wear, appearance, reflectivity, adherence of coatings and lubricant retention, to name a few. As a result, there exist a number of commercially available instruments for the measurement and assessment of surface roughness.
One common type of such instrument is the dynamic gage. The gage, which includes a frame and a movable stylus, is adapted to rest on the surface to be examined, with the stylus maintained in contact with the surface. The stylus is moved by a drive motor along a line on the workpiece surface and it follows the surface irregularities along that line. The vertical movements or vibrations of the stylus are detected and a corresponding electrical analog is developed whose amplitude represents the surface roughness along that line of the workpiece. That output signal is then averaged and used to provide a visual indication of the roughness characteristic. Usually a skid is mounted adjacent to the stylus and rides along the surface being examined to filter out the more gradual variations in the workpiece topography that contribute to its waviness. Thus, by proper selection of the skid contact area and suitable output signal filtering, one can select a particular roughness range or window to suit the particular application. In other words, low wave variations in the surface topography which do not contribute to surface roughness and very high frequency surface irregularities which are too small to be of interest may be eliminated from the measurement. Conventional surface roughness measuring instruments of this type are disclosed, for example, in U.S. Pat. Nos. 2,733,598 and 4,106,333.
Prior gages of this kind are disadvantaged for several reasons. Some are overly large and awkward to use; some are quite complex and expensive, costing two thousand dollars or more. Those instruments that are relatively compact still cannot be positioned to measure roughness at certain rounded or irregular workpiece surface, at least not without special staging fixtures. These separate fixtures can be misplaced easily and the interface between the fixture and the gage provides a potential source of measurement error. Furthermore, the fixtures which adapt the prior gages to measure rounded surfaces space the gage at different distances from the workpiece surface depending on the curvature of that surface, thereby introducing locational variations in the roughness measurements of those different surfaces. In still other conventional surface roughness gages, the gage profile, size or shape make it difficult, if not impossible, to take measurements of some relatively inaccessable surfaces.
Another problem arises with these instruments because the output signal frequency of each gage is determined by the speed of the measurement, i.e. the speed across the workpiece surface of a motor-driven stylus. Therefore, the measurement must be taken at constant speed. In some prior gages, the surface roughness data is taken based on fixed clock or recorder pulses so that motor speed variations affect the repeatability of the measured data.